Analog Integrated Circuit Device Data

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18

Freescale Semiconductor

34844

FUNCTIONAL DESCRIPTION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

POWER OFF AND POWER ON LED CHANNELS

The 34844 allows the user to Power OFF and Power ON any channel independently thru I2C/SM-BUS mode.

The POWER ON function reconnects the LED driver and the feedback circuit to the channel to allow functionality to that channel again.

On an opposite way when the channel is POWER OFF, the LED driver and feedback circuit are disconnected to the channels.

This function is very useful for applications where one or more channel has to be shutdown to avoid the output voltages goes to OVP

during the start up of the part.

The sequence to make these functions work is the following:

To POWER ON LED channels:

1. Take PWM pin low

2. Set POWER ON bit high (MSB of Register 09)

3. Set high all Channels that should be power on by writing “1” on CHENx bits (Registers 08 & 09)

4. Clear POWER ON bit

5. Take PWM pin high

To POWER OFF LED channels:

1. Take PWM pin low

2. Set POWER OFF bit high (MSB of Register 08)

3. Clear all Channels that should be power off by writing “0” on CHENx bits (Registers 08 & 09)

4. Clear POWER OFF bit

5. Take PWM pin high

POWER ON bit and POWER OFF bits shouldn’t be set at the same time to avoid damage to the part.

POWER ON/OFF channels should be reconfigured every time the part recovered from a POR or shutdown condition. This also applies

if the part is reenabled by software.

If the part is reenabled by software, it is recommended to take PWM pin low, reenable the part, then follow the corresponding sequence

shown above.

DISABLING LED CHANNELS

The 34844 allows the user to enable and disable each of the 10 channels separately by writing the corresponding CHENx bit on

Registers 08 and 09 thru I2C.

Since the enable and disable functions reconnects the feedback circuit of the LED drivers, this shouldn’t be used on any channel that

shuts down, because an open LED channel condition or because is was previously POWER OFF. This could cause instability issues,

since the voltage on this open LED driver is not substantially above the DHC regulation voltage (0.75 V typ) and may interfere with the

operation of the dynamic headroom control (DHC), leading to erratic output voltage regulation

FAIL PIN

If an LED fails to open in any of the LED strings, the voltage in that particular LED channel is close to ground and the LED open failure

is detected. When this happens, a failure is registered, the FAIL pin is set to its high-impedance stage, and the channel is shutdown.

The FAIL pin cannot be cleared for Manual mode unless a complete power on reset is applied.

However for I2C/SMBUS mode, the FAIL pin can be cleared by cycling the clear fail bit (CLRFAIL bit = 0 - 1 - 0). This allows the user

to waive any known failure and set the device to able to detect any other failure during operation.

If the fail pin cannot be cleared by software, it indicates the failure is because of an overcurrent in the Boost. Since this is a critical

failure, the only way to clear it is by releasing the part from the overcurrent condition and shutting down the part (Refer to Table 5)

If I2C communication is not present, the FAIL condition should be reset by removing the failure and re-enabling the device thru the EN

pin.

OPTICAL AND TEMPERATURE CONTROL LOOP

The 34844 supports both optical and temperature loop control.

The LED brightness can be adjusted for temperature loop control, depending on the temperature of the LEDs.

For optical loop control, the 34844 supports both optical closed loop backlight control, where the brightness of the backlight is

maintained at a required level by adjusting the light output until the desired level is achieved, or with ambient light control, where the

backlight brightness increases as ambient light increases.

Both temperature and optical loops are supported through the PIN and NIN pins. Each pin supports a 0 V to VSET (2.048 V typ.) input

range which affects the current through the LEDs. The PIN pin increases current as the voltage rises from 0 to VSET. The NIN pin reduces

current as the voltage rises from 0 - VSET.